For example, in a mobile communication system employing the W-CDMA (Wide-band Code Division Multiple Access) system, a radio signal from an apparatus at a base station must reach an apparatus at a mobile station which is physically distant. For this reason, the signal must be greatly amplified by an amplifying apparatus at the base station, and amplification is performed using even a range in which non-linear characteristics attributable to saturation can appear. Measures are therefore taken to suppress a distortion signal generated by non-linear characteristics and the like.
Recently, the pre-distortion method that allows efficient amplification is frequently used as a method for suppressing such a distortion signal. FIG. 3 is a block diagram schematically showing a distortion compensation amplifying apparatus according to the related art employing the pre-distortion method. An amplitude compensation value a and a phase compensation value b associated with each level of an input signal SIN are stored in a distortion compensation table 32 in the form of, for example, a complex amplitude (vector). A level (power or amplitude) of an input signal SIN is detected by a level detection unit 31, and an address signal A associated with the detected value is sent to the distortion compensation table 32. The address signal A specifies the addresses of an amplitude compensation value a and a phase compensation value b to be read from the distortion compensation table 32.
A pre-distorter 33 imparts a change to each of the amplitude and phase of the input signal SIN according to a pre-distortion control signal output from the distortion compensation table 32. Therefore, the amplitude compensation values a and phase compensation values b in the distortion compensation table 32 may be set such that the changes are imparted to the input signal SIN as changes having characteristics inverse to distortion of the amplitude and phase of the signal or distortion characteristics generated at each level of the input to the amplifier 34. Thus, distortion components are eliminated from an output signal SOUT output by the amplifier 34, which makes it possible to suppress leakage of power from the signal band or interference power to any adjacent channel. Hereinafter, changes imparted to the input signal SIN by the compensation values will be referred to as pre-distortions.
While the amplifier 34 in FIG. 3 is an amplifier for a radio frequency signal band, the distortion compensation table 32, a control unit 35, and so on may be digital circuits, and the signal SIN input to the pre-distorter 33 and the level detection unit 31 may be a signal in either radio frequency band or intermediate frequency band. Therefore, a frequency converter and an A-D or D-A converter are added to the circuit shown in FIG. 3 in practice depending on the configuration of those circuits. Since such variation in the circuit configuration does not relate to the invention, FIG. 3 shows a fundamental configuration only.
The characteristics of the amplifier 34 undergo changes attributable to aging and temperature changes. Unless the amplitude compensation value a and the phase compensation value b in the distortion compensation table 32 are varied in associated with such changes, distortion compensation on a pre-distortion basis cannot be properly performed. The control unit 35 acquires at least an output signal from the amplifier 34 or an evaluation value of residual distortion included in the output signal as a feedback signal and updates the compensation values in the distortion compensation table according to changes in the characteristics of the amplifier 34 to maintain optimum compensation values.
Although it is desirable that the adaptive updating of the distortion compensation table 32 performed by the control unit 35 quickly converges at optimum values, quick convergence is difficult to achieve for the reasons described below.
FIG. 4 is a graph showing an example of input-output characteristics of the amplifier 34. As shown in FIG. 4, non-linearity of the input-output characteristics of the amplifier appears more significantly as the input level becomes higher and closer to the region where the output saturates. It is therefore necessary to acquire feedback signals of output signals having great amplitudes into the control unit 35 and reflect the signals in the distortion compensation table. However, the probability of generation of, for example, W-CDMA signals having great amplitude is low, and they will be only instantaneously generated.
For example, let us assume that there are four W-CDMA carriers. Then, the signal band is 20 MHz, and an average digital device cannot perform real-time processing if sampling signals of about 100 MHz are used to deal with third- and fifth-order distortions further. When feedback signals are acquired only intermittently by stopping the acquisition of feedback signals while the control unit 35 is detecting distortion components from a feedback signal that the unit has acquired, the probability of acquiring feedback signals of output signals having great amplitude is further decreased.
Under the circumstance, various studies are being made to achieve quick convergence of compensation values.
For example, the “non-linear distortion-compensation transmitter for correcting and interpolating distortion compensation coefficients” disclosed in Patent Document 1 is equipped with distortion compensation coefficient correction means. Thus, when a distortion compensation coefficient associated with a certain input signal level is significantly different from a distortion compensation coefficient associated with another level in the neighborhood of that level, a correction process is performed to replace the deviating distortion compensation coefficient with an average of values in the neighborhood or the like. What is intended is to reduce the time require for a process of updating distortion compensation coefficients with reference to the power of distortions of transmitted outputs. The document also proposes a configuration intended for a reduction of the memory amount of a distortion compensation table, in which only distortion compensation coefficients associated with discrete input signal levels are stored and updated and in which distortion compensation coefficients associated with levels unlisted on the table are generated from the values on the table using interpolation. Referring to table updating according to Patent Document 1, an error between an input signal and an output signal is obtained, and a coefficient which nullifies the error is calculated using clipped LMS (Least Mean Square) algorithm.
In a “distortion compensation apparatus” disclosed in Patent Document 2, input signal levels are divided into a plurality of blocks, and each block is sequentially picked up to update compensation values associated with the input levels included in the block using the perturbation method such that distortions are eliminated from the transmission outputs. When the block including the greatest input signal level is updated, the update process is performed using only distortions of transmission outputs associated with input signal levels exceeding a predetermined value. This is a consideration for transmission signals which are at a peak value or a value near the peak only for a small percentage of time, i.e., transmission signals using a multi-code according to the CDMA method or multi-carrier transmission signals according to the OFDM (Orthogonal Frequency Division Multiplexing) method. Specifically, since small input signal levels result in small distortion outputs, the updating of a compensation value associated with a great input level will take a long time to reach a proper compensation value if the updating process is performed using distortions associated with arbitrary input levels, and the accuracy of resultant compensation values on the table will be low. Therefore, when the block including the greatest input level is updated as described above, the updating process is performed only when there are inputs exceeding a predetermined value and is omitted for small input levels, which allows convergence at high speed and high efficiency.
In the case of a “power amplifier” disclosed in Patent Document 3, feed forward type distortion compensation is performed with control over amplitude and phase matching in a distortion detecting loop or distortion eliminating loop stopped when a residual distortion is equal to or less than a threshold. As a result, quick convergence can be achieved, and stable distortion compensation can be maintained after the convergence is achieved.
Patent Document 1: JP-A-2002-223171
Patent Document 2: JP-A-2003-78360
Patent Document 3: JP-A-2003-87065